Does the Outback 4S store offer installation of metal skid plates, such as for the engine and transmission?

It's not necessary to install a metal skid plate. Adding one can actually affect safety. In the event of a severe collision, it might interfere with the engine's ability to drop downwards, increasing the risk of it intruding into the cabin.

Don't 4S shops provide them? Do we have to buy them outside? Just asking.

Buy it yourself from Taobao, then find a repair shop or store to install it for you. Just pay a little for labor.